1. Field
The present invention relates to disk drives for computer systems. In particular, the present invention relates to a disk drive controlling a voice coil motor during an emergency unload.
2. Description of the Related Art
A disk drive typically comprises a disk rotated by a spindle motor and a head actuated over the disk by a voice coil motor (VCM) in order to write/read data to/from the disk surface. When the spindle motor spins down the disk, for example, just prior to the disk drive being powered down or entering an idle mode, the head may be unloaded off the disk onto a ramp that extends over an outer diameter of the disk. When the disk drive is powered on or exits the idle mode, the spindle motor spins up the disk and the head is loaded off the ramp onto the disk.
During normal unload operations, an external supply voltage (e.g., from a host) provides sufficient power to enable the VCM to successfully park the head on the ramp. However, if a power failure occurs while the head is over the disk, the external supply voltage is lost, thereby requiring some other source of power to unload the head. Prior art disk drives have typically used the back electromotive force (BEMF) of the spindle motor to power the VCM during an emergency unload. A power capacitor has also been employed as a crude battery to augment the BEMF voltage, particularly in small form factor disk drives where the momentum of the disk and spindle motor may not provide sufficient energy alone to unload the head. Larger power capacitors provide more energy for unloading the head, but increase the expense of the disk drive.
There is, therefore, a need to improve the emergency unload operation of a disk drive.